Process for producing durene



No Drawing. Application June 23, 1955 Serial No. 517,643

2 Claims. Cl. 260-674) This invention relates to a process for producing durene j from hydrocarbon fractions boiling in the range from about 360 to 405 F. and having a substantial content ofisomeric tetramethylbenzenes and, more particularly, to a process for obtaining increased yields of durene in which,durene is crystallized from su'chfractions, crystallization mother liquors are isomerized in vapor phase and,

United States Patent'O 2,874,200 Patented Feb. 17, was

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form one or more tetramethylbenzenes, but rather that a substantial portion of these constituents undergo disproportionation to compounds having boiling points substantially removed from the boiling point of durene.

Thus, it was found that fractionating isomerized mother "liquors to obtain fractions boiling substantially in the additional durene is crystallized from fractions of mother liquors boiling in the range from about 360 to 405 F.

Commercially feasible durene crystallization processes generally require crystallizer feeds containing at least about 5 weight percent of durene, which feeds can be obtained by fractional distillation of petroleum reformates or aromatic alkylation products to procure relatively narrow boiling durene containing cuts. Since a substantial portion of the durene can be separated from these fractions by a single crystallization, the durene content of crystallization mother liquors is quite low and thus repeated crystallization processing of mother liquors generally. does not result in commercially significant yields of durene. However, according to the process of my invention, durene crystallization mother liquors are a source of commerciallysignificant yields of durene, and in most instances a 75 percent increment of durene yield is oh- I tainable.

Surprisingly, it was found that mother liquors from the crystallization of durene from polyalkylated aromatic hydrocarbon fractions, such as reformates and aromatic 1 alkylation products, containing a substantial portion of t tetramethylbenzenes and boiling in the range from about 360 to 405 F., can, under certain controlled conditions for reaction, be isomerized in vapor phase in the presence of a silica-alumina catalyst to obtain crystallizer feed fractions from which significant yields of durene can be obtained. In the isomerization step'of my process, more fully described hereafter, ring isomerization of methyl groups of tetramethylbenzenes such as isodurene and l prehnitene present in durene crystallization mother liquors resulting in the formation of additional durene, can be maintained the predominant reaction affecting tetramethylbenzenes, cracking and disproportionation of tetramethylbenzenes being significantly retarded. The term ring isomerization as used herein refers to shifting of intact methyl groups along and around the benzene ring of a tetramethylbenzene as distinguished from disproportionation which includes rupturing a side chain bond with relocation of the alkyl radical on a difierent ring. Surprisingly, it

range of the original feed, yields crystallizer feeds from which substantial'yields of durene can be obtained.

The production of durene, according to the process of my invention, is best illustrated by reference to its preferred form in which an aromatic concentrate such as a petroleum reformate boiling in the range from about 370 to 405 F. and consisting essentially of polyalkylated aromatic hydrocarbons including from about 15 to 25 weight percent durene and a total tetramethylbenzene content of at least about 40 percent is cooled to a temperature' below about F. to cause precipitation of crystals composed substantially of durene.- The mother liquor is separated from durene crystals by suitable means such as by filtration or centrifugation. The mother liquor is contacted in the vapor phase with-a silica-alumina catalyst at about 0.1 to 20 -W. H. S. V. and a temperature within about '50 F. of the temperature obtained from the formula:

- i'sT7-1o W. HTS. V. 460

obtaining substantially isomerization equilibrium yields of durene at the selected weight hourly space velocity, 1W. H. S. V. The isomerized mother liquor is fractionated to obtain a cut boiling in the range from about 370' to 405 F. and durene is removed from thiscut according to the crystallization technique previously mentioned. The second mother liquor from the second crystallization of durene canbe isomerized in vapor phase as before, and isomerized mother liquor can then be fractionated to obtain another substantially 370 to 405 F. durene-rich fraction from which durene can be crystallized. The process as described can be continued until the concentration of durene in a substantially 370 to 405 F. fraction of isomerized mother liquor is less than the concentration required for obtaining significant yields of durene on further'crystallization. Of course, substantially exhaused mother liquor can be further processed by the addition of fresh feed to increase total tetramethylbenzene content, and recycled, for example, to the crystal- 1 lization zone.

As was previously indicated, it was found according to the present invention, that mother liquors from durene feed or space velocity, a certain range of temperatures exist, within which range of temperatures ring isomerization of methyl groups is the predominant reaction affectmined components do not adversely affect the ring isomerization of methyl groups of tetramethylbenzenes by causing to any significant degree undesirable side' reactions. Neither does it appear that components in the feed other than tetramethylbenzenes are, isomerized to ing tetramethylbenzenes, and that at operating temperatures outside this range, either higher or lower temperatures, ring isomerization decreases rapidly. It has been determined that the relation between temperature and space velocity is expressed by the formula set forth in the preceding paragraph. Also, it was determined that ity isomerizationtemperatures were within about F. and preferably within 50 F. either higher or lower of the temperature Tm. Isomerization space velocities of the present invention range from about 0.1 to 20 while corresponding temperatures are about from 550 to 11-00 F. and the conditions are preferably equivalent in severity to about 800 to about 850 F. at 3.5 W. H. S. V.

Surprisingly, it was also observed that fractions of durene crystallization mother liquors consisting essentially of C aromatics but containing relatively minor amounts, for example, less than 2 percent, isomeric tetramethylbenzenes were subjected to the isomerization process of the present invention, the percent of tetramethyl benzenes in reactionproducts did not appreciably increase. Thus, it seems that under the controlled isomerization conditions of the present invention, only tetramethylbenzenes are isomerized to form other isomeric tetramethylbenzenes. -It follows, therefore, thatithe percentof tetramethylbenzenes in the isomerization reactionprodnets is directly proportional to their percentage in the feed stock employed. This is illustrated by the following example.

EXAMPLE I A durene crystallization mother liquor boiling. in the range from about 362 to 373 F., and consisting of about2 percent or .less by weight tetramethylbenzenes, about 5 percent methylhydrindenes, 60 percent dimethylethylbenzene and 30 percent non-aromatic and C and higher aromatic constituents, was vaporized and passed through a reactor containing a silica-alumina catalyst, composed of 90 percent silica and IOpercent alumina, at a weight hourlyuspace velocity of 0.49 and a temperature of 698 F. The temperature Tm, for obtaining substantially isomerization equilibrium yields of isomeric tetramethylbenzenes at 0.49 W. H. S. V. is about 705' F. An analysisof reaction products revealed no substantial increase in tetramethylbenzene content.

The following examples are illustrative of the functional relation between reaction temperature and-space velocity, W. H. S. V.,--in my process for the vapor phase isomerization of methyl groups of tetramethylbenzenes utilizingsilica-alumina catalysts.

EXAMPLE II A durene crystallization mother liquor boiling within the range from about 362 to 395 F. having an aromatic content of about '95 'to 98 percent and consisting of about 30 percent tetramethylbenzenes (less than 2 percent durene), 15 percent methylhydrindenes, 30 percent dirnethylethylbenzene and percent non-aromatics and C and higher aromatic constituents was vaporized and passed through a reactor containing a 90 percent silica-l0 percent alumina catalyst at atmospheric pressure and, at 0.45 W. H. .S. V. The temperature within the reactor was about 696 F. An analysis of reaction products indicated no appreciable change in total tetramethylbenzene content over that of the feed stock; however, a substantial increase in the amount of durene present to 9.1 percent, based on total weight offeed, was observed.

EXAMPLE III tion temperatures employed at substantially the same space velocity. At 0.45 W. H. S.'V., the'temperature' Tm for obtaining substantially isomerization yields of isomeric tetramethylbenzenes is about 700' F. The substantial increase in durene contentpfrom less-than 2 percent in feed to 9.1 'percent'in reaction products when isomerization was conducted at 0.45 W. V. and

' vents.

696 F. is thus due to the fact that the process was conducted substantially under operating conditions which produce isomerization equilibrium yields of tetramethylbenzenes. On the other hand, when isomerization was conducted at 0.45 W. H. S. V. and a temperature of 503 F., about 300 F. from the temperature Tm, only a slight increase in durene content occurred.

Similarly, when the isomerization process was conducted with a 362 to 395 F. durene crystallization mother liquor similar to that of Example II at 3.7 W. H. S. V. and a temperature of 949 'F., reaction products analyzed 9.2 percent durene. When the process was repeated at a temperature of 698 F. reaction products analyzed but 6.0 percent durene. Isornerization temperature Tm for 3.7 W. H. S. V. is about 840 R, which accounts for the excellent results obtained at 949 F. and the much poorer results obtained at 698 F.

.Yield of durene by my process depends to aplarge .degree on the tetramethylbenzene content of the original feed .to the crystallizer. Suitable feed :stocks include polyalkylated aromatic hydrocarbon fractions-boiling in thevrange from about 360 to 405 Rand .con-

taining at least about 20 weight percent of tetramethylbenzenes including at least about 5 weight percent of .durene. Because of the isomerization equilibrium that exists between tetramethylbenzene isomers, only a .por-

tion of isodurene and prehnitene are converted vto durene during mother liquor isomerization, and thus crystallizer feedshaving a total tetramethylbenzene content of less thanabout 20 percent generally do not produce isomerized mother liquor fractions having a durene content sutficient for obtaining significant yields of durene. A'preferred crystallizer feed is a petroleum reformate fraction boiling in the range from 370 to 405 F. and

containing about 40 weight percent tetramethylbenzenes including at least about 15 weight percent durene.. Al-

vso, fractions boiling within about this range obtained from products resulting from the alkylation of aromatic hydrocarbons, as for example, those products resulting from the alkylation of xylenes with methyl chloride, are excellent .crystallizer feeds.

Separation of durene from an original charge stock and isomerized mother liquor fractions is accomplished by crystallization techniques generally familiar to-the petroleum industry. ,The temperature to which 'a .durene-containing hydrocarbon fraction must be chilled in order to precipitate durene in a crystalline or solid state is dependent upon the concentration .of durene in the particular crystallizer feed, the greater the durene content the higher the crystallizationtemperature which can be employed. Crystallization temperatures range from about -l00 F. for crystallizer feeds containing about 5 percent durene to about -20 F. for feeds containing about,50 percent durene. Of course, crystallization temperatures exist intermediate of and above those specifically mentioned and depend upon durene concentration in crystallizer charge stock. Mother liquors are generally separated from durene crystals .by centrifugation or filtration.

The crude durene cake can be purified by recrystallization either with or without the employment of sol- In a preferred form of the invention-a hydrocarbon fraction similar to the preferred crystallizer feed mentioned above containing about 15 percent durene, is chilled to a temperature below about -50 F. and durene crystals are separated from the mother liquor by centrifugation. The crude cake is melted and then cooled ,to about 20 to 30 F. at which temperature the .newly formed cake is filtered and washed with a suit- .able solvent such as acetone.

also causes the disproportionation of a'substantial portion of non-tetramethylbenzene constituents present in the durene crystallization mother liquors to compounds having boiling points substantially removed from the boiling point of durene. Thus, by fractionating isomerized mother liquors to obtain fractions boiling in the range from about 360 to 405 F. crystallized feeds can be obtained having durene concentrations greater than about 5 percent and generally having concentrations of durene from about to percent. Isomerized mother liquors are preferably fractionated so as to obtain a cut boiling in the range from about-370 to 405 F.

Catalysts for the present invention are of silica-alumina composition and include synthetic composite gel catalysts comprising a major portion of silica and a minor portion of alumina. Catalysts of thistype are well known and widely used in the oil industry for the conversion or cracking of hydrocarbon oils. They ordinarily contain from about 50 to 95 percent of silica and from 50 to 5 percent of alumina. A preferred catalyst is one composed of 90 percent silica and 10 percent alumina. Of course, the catalyst may contain minor amounts of other materials; and catalyst diluents, such as tabular alumina, may be present in the catalyst bed so long as they do not deleteriously affect the isomerization process.

The obtaining of substantial additional yields of durene according to the process of my invention will be clearly evident from the following examples thereof, which examples are given merely by way of illustration and are not to be construed as limiting the scope of my invention. In each of the examples, all percentages are based on total weight of original feed to the first crystallization step unless otherwise indicated.

EXAMPLE IV A substantially C aromatic concentrate boiling at about 370 to 395 F. having a total tetramethylbenzene content of about 52 percent and containing about 22 percent durene, 8 percent methyl hydrindenes, percent dimethylethylbenzenes, and 20 percent non-aromatic and C and higher aromatic constituents was chilled to -100 F. and a yield of 19.9 percent durene in crystalline form was obtained after separation of mother liquor by filtration. The mother liquor containing about 2.1 percent durene was vaporized and passed through a reactor containing a silica-alumina catalyst, composed of about 90 percent silica and 10 percent alumina, at substantially atmospheric pressure and at a temperature of 700 F. and 0.5 W. H. S. V.

. Freezing point data indicated the isomerized mother liquor to contain about 10.9 percent durene. The isomerized mother liquor was fractionated to remove constituents boiling above 404 F. and then was chilled to -100 F. A yield of 7.2 percent durene as crystals was obtained after separation of a second mother liquor by filtration. The second mother liquor was vaporized and contacted with a silica-alumina catalyst under the conditions previously described. The second isomerized mother liquor analyzing 6.9 percent durene was fractionated to obtain a cut boiling below 404 F. This cut was chilled to l00 F. and a yield of 4.2 percent durene crystals was obtained. The third mother liquor was subjected to the isomerization, fractionation and crystallization steps described above and a yield of 1.7 percent durene crystals was obtained from isomerized mother liquor analyzing 4.6 percent durene. The process was again carried out using the mother liquor from the previous durene crystallization step and a yield of 0.9 percent durene was obtained from isomerized mother liquor analyzing 5.6 percent durene based on feed to the last isomerization stage.

EXAMPLE V A substantially C aromatic concentrate boiling at about 370 to 404 F. having a total tetramethylbenz ene content of about 45.2 percent and containing about 17.6 percent durene, 12.6 percent methylhydrindenes, 16.9 percent dimethylethylbenzenes, and 25.3 percent non-aromatic and C and higher aromatic constituents was chilled to 100 F. and a yield of 15.7 percent durene in crystalline. form was obtained after separation of mother liquor by filtration. The mother liquor containing about 1.9 percent durene was vaporized and passed through a reactor containing a silica-alumina catalyst, composed of about 90 percent silica and 10 percent alumina, at substantially atmospheric pressure and at a temperature of 850 F. and 3.7 W. H. S. V. Freezing point data indicated the isomerized mother liquor to contain about 10.2 percent durene based on feed to this stage. The isomerized mother liquor was fractionated to obtain a substantially 370 to 404 F. cut which was chilled to -100 F. A yield of 5.4 percent durene was obtained as crystals after separation of a second mother liquor by filtration. The second mother liquor was combined with light ends and bottoms obtained from the fractionation of isomerized first mother liquor and was vaporized and contacted with a silica-alumina catalyst under the conditions pre -viously described. The second isomerized mother liquor analyzing 8.2 "percent durene based on feed to this stage was fractionated to obtain a cut boiling at about 370 to 404 F. This cut was chilled to 100 F. and a yield of 4.2 percent durene crystals was obtained. The third mother liquor was combined with light ends and bottoms as before and was subjected to the isomerization, fractionation and crystallization steps described above and a yield of 2.5 percent durene crystals was obtained from isomerized mother liquor analyzing 6.7 percent durene based on feed to this stage.

Referring to Example IV, an increment of 71 percent durene, based on weight of durene originally crystallized from a 370 to 395 F. cut, wasrobtained by recycling mother liquors four times to the isomerization step and separating durene from fractions of isomerized mother liquor boiling below about 404 F. In Example V, a 77 percent increment in durene yield was obtained from a 370 to 404 F. crystallizer feed stock by recycle of mother liquors three times. Table I shows the increment obtained by each recycle of mother liquor according to my process.

Table 1 Total Percent Increment Number of Times Recycled 370-395 F. 370-404 F. Feed Feed Table II Durene, Wt. Percent Durene, 370-404 F. Wt. Percent of Product Number of Times Recycled From Table II, it is apparent that according to my process durene is concentrated in the 370 to 404 F. fraction of: isomerized mother liquor by. disproportionation of non-tetramethylbenzene. constituents. to compounds; boilingoutside thisufange; Thus, crystallizer-feed: stocks-having na; substantial v durene .contentv from. which commercial yieldsv of'durenecan. be produced are obtained according tomyzprocess.

Iclaim:

l; A process. forproducing. durener which: comprises chillingahydrocarhon fraction boiling in the range; from aboutz36l1- to.4.05.. F. and consisting essentially-of:polyalkylated aromaticzhyd-rocarbons including at; least about 5 weightpercentof;dureneand-having atotal tetrarnethylbenzene; content of. at. least about-.20 weight percent to crystallize; durene= in said. fraction, separating mother liquor: fromv durene crystals, contacting said, mother liquor in vapor phase with a, silica-aluminacatalyst atabout 0.1 to 20 W. H. S. V.. and; a, temperature within about 100'=E. of the temperature: obtained from the formula:

in -wh-ich Tm;is the temperature in degrees F. for obtainingsubstantially. isomerization equilibrium yields of dureneat the selected W. H. S. V., fractionating theisd' in which Tmis the temperature indegrees F. for obtain: ing substantially isomerization. equilibrium yields of du-, rene. at. the selected. W. H- S. V., fractionating the isomerizedmother, liquor. to obtain a, fraction boiling inthe range from about'370 to 405. F., chilling said fraction of. isomerized. motherv liquor to crystallize durene, and separating durene crystals from said isomerized mother liquor.

References Cited in the file of this patent UNITED STATES PATENTS Boedeker et al. Sept. 27, 1955 Fetterly July 24, 1956 

1. A PROCESS FOR PRODUCING DURENE WHICH COMPRISES CHILLING A HYDROCARBON FRACTION BOILING IN THE RANGE FROM ABOUT 360* TO 405*F. AND CONSISTING ESSENTIALLY OF POLYALKYLATED AROMATIC HYDROCARBONS INCLUDING AT LEAST ABOUT 5 WEIGHT PERCENT OF DURENE AND HAVING A TOTAL TETRAMETHYLBENZENE CONTENT OF AT LEAST ABOUT 20 WEIGHT PERCENT TO CRYSTALLIZE DURENE IN SAID FRACTION, SEPARATING MOTHER LIQUOR FROM DURENE CRYSTALS, CONTACTING SAID MOTHER LIQUOR IN VAPOR PHASE WITH A SILICA-ALUMINA CATALYST AT ABOUT 0.1 TO 20 W. H. S. V. AND A TEMPERATURE WITHIN ABOUT 100*F. OF THE TEMPERATURE OBTAINED FROM THE FORMULA: 